The surface stabilized ferroelectric liquid crystal (SSFLC) light valve has been shown to possess properties useful in a number of opto-electronic device applications requiring high contrast ratio. These include electro-optic shutters, spatial light modulators for opto-electronic computing, and flat panel display devices. In such devices, the speed of response is often important. This response speed is given approximately by equation 1: ##EQU1## wherein .tau. is the optical response (10%-90%) to an applied voltage step of magnitude E, .eta. is the orientational viscosity, and P is the ferroelectric polarization density.
The physics and operation of the SSFLC has been extensively described (Clark, N. A. et al. (1983) Mol. Cryst. Liq. Cryst. 94:213; Clark and Lagerwall U.S. Pat. No. 4,367,924; Clark and Lagerwall U.S. Pat. No. 4,563,059). In the surface stabilized state, FLC molecules lie in layers perpendicular to the glass plates (the so-called bookshelf geometry). The FLC optic axis makes an angle .+-..theta. with respect to the layer normal. For many mixtures, .theta.=.+-.22.5.degree., so the FLC cell acts like a retarder which can be electronically rotated by 45.degree.. The voltage requirements for such switching devices are modest (.+-.10 V), and power consumption is quite low because the voltage need not be applied to maintain the FLC in the switched state: the devices are bistable (Clark, N. A. and Lagerwall, S. T. (1980) Appl. Phys. Lett. 36:899). Typical switching times are &lt;44 .mu.s at room temperature (ZLI-3654 mixture available from E. Merck, D-6100 Darmstadt 1, Frankfurter, Strabe, 250, F.R.G.). Several other alignment configurations for FLC cells have been described (Clark and Lagerwall, U.S. Pat. No. 4,563,059).
The contrast (ratio of transmitted light intensity through the cell in the bright and dark states) in the standard SSFLC cell is greatest when the tilt angle .theta. of the FLC material is 22.5.degree.. Under these conditions, at the half wave thickness (where d=.lambda./2.DELTA.n) between crossed polarizers (an entrance polarizer and an exit polarizer or analyzer) the dark state will leave polarization of the input light unchanged, while the bright state will rotate the plane of polarization of the input light through 90.degree.. In general, in the on (switched) state the plane of polarization of the output light will be rotated through 4.theta., where .theta. is the tilt angle.
The orientation viscosity .eta. in FLC mixtures generally increases with increasing tilt angle. Often, .eta. increases with tilt angle faster than P, and thus materials with low tilt angle (i.e. .theta.&lt;15.degree.) often show improved electro-optic response speed relative to similar materials with 22.5.degree. tilt angle. However, this increase in speed is achieved at the expense of throughput, since the output light in the SSFLC is rotated through &lt;90.degree., and a significant amount of the light in the on state is extinguished at the analyzer.
Light valves based upon the electroclinic effect occurring in chiral smectic A* FLC materials exhibit several attractive features (see Andersson et al. (1987) Appl. Phys. Lett. 51:640), including very fast response and voltage regulated gray scale. The electroclinic effect is related to the variation in the birefringence of a material as a function of an applied electric field (see Garoff and Meyer (1977) Phys. Rev. Lett. 38:848). A number of chiral smectic A* materials have been shown to display an electroclinic effect when incorporated into SSFLC type cells. The applied voltage induces or varies the tilt angle in these materials in an analog fashion. The effect is described as being linear in applied voltage with very rapid response. However, for all currently known materials, the maximum tilt angle achieved due to the electroclinic effect is small (i.e. .theta.&lt;17.5.degree.).
The distorted helix ferroelectric effect has been described with smectic C* liquid crystals having a short pitch (see Ostrovski and Chigrinov (1980) Krystallografiya 25:560 and Ostrovski et al. in Advances in Liquid Crystal research and Application, (l. Bata, ed.) Pergamon, Oxford; Funfschilling and Schadt (1989) J. Appl. Phys. 66:3877). In SSFLC cells incorporating the shortpitch materials, the helix of the material is not suppressed, and thus the helix can be distorted by the application of an electric field. This distortion results in a field dependent change in the tilt angle of the material. DHF materials also display voltage-dependent variations in birefringence. DHF cells are attractive since high induced tilt angles (up to .+-.38.degree.) can be attained with applied voltages lower than those required for smectic A* cells. Beresnev et al., EPO Patent Application published Apr. 5, 1989, described FLC cells incorporating DHF materials.
Birefringent filters were first used in solar research where sub-angstrom spectral resolution is required to observe solar prominences. The first type of birefringent filter was invented by Lyot (Lyot, B. (1933) Comptes rendus 197:1593) in 1933. The basic Lyot filter (Yariv, A. and Yeh, P. (1984) Optical Waves in Crystals, Chapter 5, John Wiley and Sons, New York) can be decomposed into a series of individual filter stages. Each stage consists of a birefringent element placed between parallel polarizers. The exit polarizer for a particular stage acts as the input (or entrance) polarizer for the following stage. In a Lyot-type filter, fixed birefringent elements are oriented with optic axes parallel to the interface and rotated 45.degree. from the direction of the input polarization. The thickness, and therefore the retardation of the birefringent elements, increases geometrically in powers for two of each successive stage in the Lyot geometry. Multiple stage devices have been demonstrated with high resolution (0.1 angstrom) and broad free-spectral-range (FSR) addressing, for example, the entire visible spectrum (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815).
More recently, research in optical filters has focused on tuning the wavelength of peak transmission. An optical filter which can be rapidly tuned has applications in remote sensing, signal processing, displays and wavelength division demultiplexing. Tunability of otherwise fixed frequency Lyot filters has been suggested and implemented using various techniques (Billings, B. H. (1948) J. Opt. Soc. Am. 37:738; Evans, J. W. (1948) J. Opt. Soc. Am. 39:229; Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815). These include mechanical methods such as stretching plastic sheets in series with the birefringent elements (Billing's, B. H. (1948) J. Opt. Soc. Am. 37:738), mechanically rotating waveplates (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815) or sliding wedge plates (Evans, J. W. (1948) J. Opt. Soc. Am. 39:229), changing the retardation of the birefringent elements by temperature tuning the birefringence, or changing the birefringence using electro-optic modulators (Billings, B. H. (1948) J. Opt. Soc. Am. 37:738). Temperature tuning and mechanical tuning methods are inherently slow. Electro-optic tuning of known filter devices, while potentially more rapid, requires large drive voltages and is limited in bandwidth by material breakdown voltages for the thin birefringent elements required (Weis, R. S. and Gaylord, T. K. (1987) J. Opt. Soc. Am. 4:1720).
Other electronically tunable filters, which have been demonstrated include acousto-optic tunable filters (hereafter AOTF) (Harris, S. E. and Wallace, R. W. (1969) J. Opt. Soc. Am. 59:744; Chang, I. C. (1981) Opt. Eng. 20:824), electro-optic tunable filters (hereafter EOTF) (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391; Lotspeich, J. F. et al. (1981) Opt. Eng. 20:830), multiple-cavity Fabry-Perot devices (Gunning, W. (1982) Appl. Opt. 21:3129) and hybrid filters such as the Fabry-Perot electro-optic Solc filter (Weis, R. S. and Gaylord, T. K. (1987) J. Opt. Soc. Am. 4:1720).
The operation of the AOTF is based on the interaction of light with a sound wave in a photoelastic medium. Strong acousto-optic interaction occurs only when the Bragg condition is satisfied. Therefore, only one spectral component of incident radiation is diffracted from the structure at a given acoustic frequency. Tuning is accomplished by changing the acoustic frequency. This was the first electrically tunable filter, which succeeded in varying the transmission wavelength from 400 nm to 700 nm by changing the acoustic frequency from 428 MHz to 990 MHz with a bandwidth of approximately 80 nm (Harris, S. E. and Wallace, R. W. (1969) J. Opt. Soc. Am. 59:744). Current AOTF's have 12.degree. fields of view, high throughput, high resolution and broad tunability (Chang, I. C. (1981) Opt. Eng. 20:824). However, power requirements are high for many applications (on the order of 10 watts/cm.sup.2) and frequency shifts induced by the filter prohibit the use of AOTF's in laser cavities.
The EOTF consists of a Y-cut LiTaO.sub.3 platelet, placed between crossed polarizers, with an array of separately addressable finger electrodes (Pinnow, D. A. et al. (1979) Appl. Phys. Lett. 34:391). Tunability is accomplished by applying a spatially periodic (sinusoidal) voltage to the 100 electrodes. Current applications of this device, however, utilize more elaborate programmable passband synthesis techniques (Lotspeich, J. F. et al. (1981) Opt. Eng. 20:830). While the power requirements for the EOTF are low, it suffers from a small clear aperture and field-of-view. This is also the main disadvantage of the Fabry-Perot devices.
Color switching has been described in liquid crystal displays which incorporate dichroic dyes (see, e.g., Aftergut et al. U.S. Pat. No. 4,581,608). Buzak in U.S. Pat. No. 4,674,841 refers to a color filter switchable between three output colors incorporating a variable retarder which is a twisted nematic liquid crystal cell. Nematic liquid crystals have also been used for tuning optical filters (Kay, W. I., U.S. Pat. No. 4,394,069; Tarry, H. A. (1975) Elect. Lett. 18:47; Gunning, W. (1980) Proc. SPIE 268:190; and Wu, S. (1989) Appl. Opt. 28:48). The main disadvantage of these is their slow tuning speed (.about.100 ms).
Clark and Lagerwall in U.S. Pat. No. 4,367,924 "Chiral Smectic C of H Liquid Crystal Electro-Optical Device" refer to color control as an attribute of their ferroelectric liquid crystal electro-optical device and state that "[the] sample birefringence and orientation of the two polarizers can be manipulated to give color effects." It appears that the exit polarizers are rotated to select color.
Clark and Lagerwall in U.S. Pat. No. 4,563,059 "Surface Stabilized Ferroelectric Liquid Crystal Devices" refer to color production using FLC layers. At least two methods of color production are discussed. The first involves using spatial multiplexing of a 2.times.2 pixel array containing FLC cells placed between polarizers to generate four colors where the FLC cells of each pixel in the array have a different thickness. The second method involves two FLC layers positioned on top of one another to give 2.times.2 colors. Specifically a device comprised of two FLC devices which are positioned such that they have a specific tilt angle of 24.degree. between the optic axes in the switched state is described for color production.
Ozaki et al. (1985) Jpn. J. Appl. Phys. (part 1) 24 (suppl. 24-3):63 refer to a high speed color switching element in which dichroic dyes are mixed with ferroelectric liquid crystals. Color switches and/or displays which combine color filters and ferroelectric liquid crystal cell shutters have been described. See, e.g., Seikimura et al. U.S. Pat. No. 4,712,874; Takao et al. U.S. Pat. No. 4,802,743; Yamazaki et al. U.S. Pat. No. 4,799,776; Yokono et al. U.S. Pat. No. 4,773,737.
Carrington et al. (1989) Second International Conference on Ferroelectric Liquid Crystals Program and Abstracts (Goteborg, Sweden, 27-30 Jun. 1989) Abstract 015 refer to rapid switching of spatial arrays of FLC two color switches in color displays.
Lagerwall et al. (1989) "Ferroelectric Liquid Crystals: The Development of Devices" Ferroelectrics 94:3-62 is a recent review the use of FLC cells in device applications. In a selection called "SSFLC Color" the reviewers refer to color display (e.g. for television applications). The reviewers refer to the production of color using a red-green-blue microfilter repetitive pattern in front of a liquid crystal and reference J. C. White 91988) Phys. Tech. 19:91. The reviewers refer to a multicolor FLC screen and reference Matsumoto et al. (1988) SID 88 Digest 41. The reviewers also refer to "color sequential backlighting" and reference J. C. White (1988) supra, and to C. M. Waters (1988) EPO Patent Application Publication No. 0 261 901.